Soil structure and bypass flow processes in a Vertisol under sprinkler and drip irrigation

In this paper morphological and physical characteristics, as well as flow behaviour of a Mediterranean Vertisol under the influence of two different irrigation systems currently used for irrigation, i.e. drip and sprinkler systems, were compared. No differences in soil texture, compaction and in potential cracking were found on cores from the two fields. However, field application of methylene blue showed the presence of continuous macropores, penetrating up to depths of 20–25 cm from the soil surface, in the field where the drip system was in use (field 1). This was considered to be the pre-existing soil structure. Instead, macropores terminating at a depth ranging between 5 and 10 cm from the soil surface were observed in the sprinkler irrigated field (field 2). The same difference in terms of macropores' continuity was also observed on soil cores sampled from the two irrigated fields. The higher raindrop impact and the non-point water application involved in the sprinkler irrigation system were assumed to have determined, during several years, the different depth of penetration of the macropores in the two fields. A different hydraulic behaviour was evidenced by laboratory measurement of bypass flow on soil cores taken from the two fields. Specifically, higher values of the saturated hydraulic conductivity were found in the cores from the drip irrigated field compared to those sampled in the sprinkler field. In addition no bypass flow was measured in the columns under the sprinkler field, while high rates and amounts of bypass flow were obtained in the cores taken from the drip irrigated field. The different hydraulic behaviour observed in the cores taken from the drip and from the sprinkler irrigated field was in agreement with the difference in terms of macropores' continuity between the two fields. Being bypass flow a mechanism inducing leaching of solutes, results of this investigation suggest that irrigation systems affecting soil structure, and altering macropores' continuity, should be avoided in clay soils.